This invention is in the field of high-speed digital data communications, and is more specifically directed to digital subscriber line (DSL) communications using discrete multitone (DMT) modulation.
Digital Subscriber Line (DSL) technology has become one of the primary technologies in the deployment of high-speed Internet access in the United States and around the world. As is well known in the art, DSL communications are carried out, using existing telephone “wire” facilities, between individual subscribers and a central office (CO) location operated by a telephone company or an Internet service provider. Typically, some if not all of the length of the loop between the CO and the customer premises equipment (CPE) is implemented by conventional twisted-pair copper telephone wire. Remarkably, modern DSL technology is able to carry out extremely high data rate communications, even over reasonably long lengths (e.g., on the order of 18,000 feet) of twisted-pair wire, and without interfering with conventional voiceband telephone communications.
Modern DSL communications achieve these high data rates through the use of multicarrier modulation (MCM) techniques, also referred to as discrete multitone modulation (DMT), by way of which the data signals are modulated onto multiple frequencies over a relatively wide frequency band (on the order of 1.1 MHz for conventional ADSL, and up to as high as 30 MHz for VDSL), this band residing well above the telephone voice band, and subdivided into many subchannels. The data symbols modulated onto each subchannel are encoded as points in a complex plane, typically according to a quadrature amplitude modulation (QAM) constellation. The number of bits per symbol for each subchannel (i.e., the “bit loading”), and thus the number of points in its QAM constellation, is determined according to the signal-to-noise ratio (SNR) at the subchannel frequency, which depends on the transmission channel noise and the signal attenuation at that frequency. For example, relatively noise-free and low attenuation subchannels may communicate data in ten-bit to fifteen-bit symbols, represented by a relatively dense QAM constellation with short distances between points in the constellation. On the other hand, noisy channels may be limited to only two or three bits per symbol, allowing a greater distance between adjacent points in the QAM constellation. High data rates are attained by assigning more bits (i.e., a more dense QAM constellation) to subchannels that have low noise levels and low signal attenuation, and loading subchannels with poorer SNRs with a fewer number of bits, or none at all.
FIG. 1 illustrates the data flow in conventional DSL communications, for a given direction (e.g., downstream, from a central office “CO” to customer premises equipment “CPE”). The input bitstream that is to be transmitted, typically a serial stream of binary digits in the format as produced by the data source, is applied to bit-to-symbol encoder 11 in a transmitting modem 10. Encoder 11 groups the bits in the input bitstream into multiple-bit symbols that are used to modulate the DMT subchannels, with the number of bits in each symbol determined according to the bit loading assigned to its corresponding subchannel, based on the characteristics of the transmission channel as mentioned above. Encoder 11 may also apply error correction coding, such as Reed-Solomon coding, for error detection and correction purposes; other types of coding, such as trellis, turbo, or LDPC coding, may also be applied for additional signal-to-noise ratio improvement. The symbols generated by encoder 11 correspond to points in the appropriate modulation constellation (e.g., QAM), with each symbol associated with one of the DMT subchannels.
The encoded symbols are then applied to inverse Discrete Fourier Transform (IDFT) function 12, which associates each symbol with one subchannel in the transmission frequency band, and generates a corresponding number of time domain symbol samples according to the Fourier transform. These time domain symbol samples are then converted into a serial stream of samples by parallel-to-serial converter 13. This serial sequence of symbol values is representative of the sum of a number of modulated subchannel carrier frequencies, with the modulation indicative of the various data values. Typically, N/2 unique complex symbols (and its N/2 conjugate symmetric symbols) in the frequency domain will be transformed by IDFT function 12 into a block of N real-valued time domain samples.
As known in the art, cyclic insertion function 14 adds a cyclic prefix or suffix, or both, to each block of serial samples presented by parallel-to-serial converter 13. In conventional ADSL, cyclic insertion function 14 prepends a selected number of sample values from the end of the block to the beginning of the block. In ADSL2+ and VDSL, cyclic prefix and suffix insertion, and transmitter windowing, are combined into a single module, such as cyclic insertion function 14. Upsampling function 15 and digital filter function 16 then process the digital datastream in the conventional manner. Digital filter 16 may include such operations as a digital low pass filter for removing image components, and digital high pass filtering to eliminate voice band or ISDN interference. The digital functions corresponding to encoder 11 through digital filter function 16 is typically performed by a digital transceiver integrated circuit, which may be implemented as a digital signal processor (DSP) device.
The filtered digital datastream signal is then converted into the analog domain by digital-to-analog converter 17. Analog filtering (not shown) may then be performed on the output analog signal, such filtering typically including at least a low-pass filter. The analog signal is then amplified by amplifier 18. Digital-to-analog converter 17, amplifier 18, and any analog filtering, may be implemented in a so-called “analog front end”, including a coder/decoder (“codec”), a line driver and receiver, and a hybrid circuit.
The resulting DMT signal is transmitted over a channel LP, over some length of conventional twisted-pair wires, to a receiving DSL modem 20, which, in general, reverses the processes performed by the transmitting modem to recover the input bitstream as the transmitted communication. A receiver “analog front-end”, typically including a corresponding hybrid circuit and line receiver, and analog filtering function 21, removes high frequency noise and aliasing from the received analog signal. Analog equalization of the signal may also be performed to compensate for line attenuation characteristics of transmission channel LP. Analog-to-digital conversion 22 then converts the filtered analog signal into the digital domain, following which conventional digital filtering function 23 is applied to augment the function of the analog filters.
Digital filter function 23 forwards the filtered digital datastream to time domain equalizer (TEQ) 24, which is typically a finite impulse response (FIR) digital filter that effectively shortens the length of the impulse response of the transmission channel LP, including the filtering that is performed prior to receipt by TEQ 14. The cyclic prefix is removed from each received block in function 25, and serial-to-parallel converter 26 converts the datastream into a number of samples (2N) for application to Discrete Fourier Transform (DFT) function 27. The DFT of this datastream will recover the modulating symbols at each of the subchannel frequencies, reversing the IDFT performed by function 12 in transmission, and presenting a frequency domain representation of the transmitted symbols multiplied by the frequency-domain response of the effective transmission channel. Frequency-domain equalization (FEQ) function 28 divides out the frequency-domain response of the effective channel, recovering the modulating symbols. Symbol-to-bit decoder function 29 then resequences the symbols into a serial bitstream, decoding any encoding that was applied in the transmission of the signal, and producing an output bitstream that corresponds to the input bitstream upon which the transmission was based. This output bitstream is then forwarded to the client workstation, or to the central office network, as appropriate for the location.
While the data flow of FIG. 1 shows communications in only one direction, each DSL transceiver (i.e., both at the CO and also in the CPE) includes both a transmitter and a receiver, and as such communicates data in the opposite direction over transmission channel LP, according to a similar DMT process. To avoid interference in this bidirectional communication over the same transmission channel LP, the most popular implementation of DSL has been asymmetric DSL (“ADSL”), which follows a frequency-division duplexing (FDD) approach in that “downstream” communications from the CO to the CPE are in one frequency band of the spectrum, and “upstream” communications from the CPE to the CO are in another, non-overlapping, frequency band. For example, downstream communications in modern ADSL occupies 256 subchannels of 4.3125 kHz bandwidth, while upstream communications use 64 such subchannels at lower frequencies than the downstream band (but still above the voice band). The asymmetry suggested by the acronym “ADSL” refers to the wider and higher-frequency band that is assigned to downstream communications, relative to the narrower, lower-frequency, upstream band. As a result, the ADSL downstream data rate is usually much greater than the upstream data rate. Typical downstream data rates for conventional ADSL communications can reach and exceed 8.0 Mbps, depending upon the loop length and channel transmissions.
Of course, the demand continues for ever-higher data rate DSL technologies. The demand for higher data rats is contemplated to escalate as such high volume services as video-on-demand and other video distribution over DSL links, video telephony, and the like are deployed. Newer DSL technologies provide higher data rates by variations of the DMT scheme of ADSL. A first higher data rate DSL approach is known as “ADSL2+”, and extended the signaling bandwidth to 2.2 MHz by doubling the number of downstream subchannels to 512, each having 4.3125 kHz. An example of this approach is described in U.S. Pat. No. 5,519,731, now commonly assigned with this application and incorporated herein by this reference. Further advances in DSL data rates, beyond ADSL2+, have now been developed. These technologies are known as “very high bit-rate DSL” (“VDSL” and “VDSL2” are exemplary classes). According to these technologies, up to as many as 4096 subchannels, covering a signaling bandwidth extending up to 30 MHz, are known.
It is becoming apparent that VDSL2 communication approaches will have different implementations in different regions of the world. For example, VDSL2 in North America and Europe is contemplated to provide data rates of up to 30 to 50 Mbps, by using a 12 MHz signaling bandwidth. In Japan and Korea, however, data rates of up to 200 Mbps are contemplated, using a signaling bandwidth of up to 30 MHz. It is believed that the difference in bandwidth and data rates between these markets results from the difference in loop lengths, with much longer distances (up to 18,000 feet) from the central office or optical network unit (ONU) to the subscriber permitted in North America and Europe. Current draft standards for VDSL2 communications contemplate these optional implementations. The primary differences between these VDSL2 approaches are realized by differences in the number of subchannels carried over a given line, and also by differences in the bandwidth allotted to each subchannel.